1. Field of the Invention
The present invention relates to an image recording method and an image recording apparatus, and more particularly, to technology for compensating non-uniformity in density caused by an image recording apparatus which records an image by forming dots on a recording medium.
2. Description of the Related Art
As an image recording apparatus, an inkjet recording apparatus (inkjet printer) is known, which comprises an ink ejection apparatus having an inkjet head (recording head) with a plurality of arranged nozzles (ejection elements), for example. According to such an image recording apparatus, images are formed on a recording medium by ejecting ink from the nozzles toward the recording medium while the inkjet head and the recording medium are caused to be moved relatively to each other.
In an inkjet recording apparatus of this kind, there is a possibility that one-dimensional non-uniformity may arises due to errors in the print characteristics (ejection characteristics) of each nozzle. In particular, in a full-line head which is capable of recording images over the full surface of a recording paper simply by performing one operation of moving the recording paper and the head relatively to each other (by performing a single-pass), the print characteristics of the nozzles are reflected directly on the recording paper. Therefore, linear non-uniformity in the conveyance direction of the recording paper is readily noticeable.
For example, as shown in FIG. 29A, an image is recorded by forming ink dots 908 on the recording paper 904 by ejecting ink droplets 906 from nozzles 902, while a recording paper 904 is conveyed with respect to a line type recording head 900 having nozzles 902 arranged in one row, in a direction substantially perpendicular to the nozzle arrangement direction (in the direction indicated by the arrow in FIG. 29A).
In this case, if, of the ink droplets 906 ejected from the nozzles 902, there is an ink droplet 906a having a displaced landing position, or an ink droplet 906b having a greater ejection volume than that under normal conditions, then a white stripe 910, a black stripe 912 of increased density, and the like, appear on the recording paper 904, thus giving rise to non-uniformity in density. As a result, linear non-uniformity appears on the recording paper 904, as shown in FIG. 29B, and this might become a problem in terms of image quality.
In this way, errors in the ejection volume or errors in the landing position in the direction perpendicular to the conveyance direction of the recording paper, lead to band-shaped lines due to the non-uniformity in density in the conveyance direction of the recording paper.
In view of such non-uniformity in density, a number of technologies have been proposed in which errors in the print characteristics of the nozzles are measured in advance and then non-uniformity in density is compensated on the basis of the errors thus measured.
Broadly speaking, two types are known as such techniques for compensating non-uniformity of density.
The first type of method should be called a physical compensation method. This method adjusts the ejection volume to a target value, by controlling the drive waveform, and the like, according to the errors in the ejection volume, and adjusts the ink-landing position to a target position by controlling the flight direction of the ejected ink, and the like, according to errors in the landing position.
Furthermore, the other type should be called a visual compensation method. This method adjusts the density within a region of a certain surface area, to a target density value, by controlling the image signal to adjust the droplet ejection rate, and thereby non-uniformity of density is compensated.
For example, a method is known in which, a solid image having non-uniformity is created as a test chart in order to compensate non-uniformity of density arising principally from ejection volume errors after optically reading in non-uniformity of density, the density characteristics of nozzles are measured by optically scanning this chart, and the droplet ejection rate is compensated so that the density characteristics of the nozzles conform to target values (see, for example, Japanese Patent Application Publication No. 5-69545).
Furthermore, in order to compensate linear non-uniformity caused by landing position errors, a method is known in which, firstly, landing position error information for each nozzle is acquired on the basis of a special test pattern, whereupon the density characteristics of the print area corresponding to a certain nozzle are estimated on the basis of the effect of the landing position errors of the surrounding nozzles (the effect of the deviation of the landing positions of the adjacent nozzles), and then the compensation is performed on the basis of the density characteristics thus estimated (see, for example, Japanese Patent Application Publication No. 2004-58282).
However, in the related art technology described above, it is difficult to completely compensate the linear non-uniformity caused by landing position errors. This is described in more detail below with reference to the drawings.
For example, in the case of a recording paper (media) 904 and a (line type) recording head 900 are disposed as shown in FIG. 30, Di represents the region (print area) on the recording paper 904 covered by the i-th nozzle 902i. The optical density of the print area Di corresponding to each nozzle 902i is measured while an optical sensor 920 is successively conveyed on the nozzle pitch basis, in the direction indicated by the arrow in FIG. 30. In this case, the aperture diameter of the optical sensor 920 in the scanning direction corresponds to the nozzle pitch.
For example, in Japanese Patent Application Publication No. 5-69545, a test pattern is printed and the optical density with respect to each area is measured, thereby acquiring the density characteristics of the nozzles. Furthermore, in Japanese Patent Application Publication No. 2004-58282, the nozzle characteristics are measured in advance, and then the density characteristics of the each area are estimated from the dot shape.
Here, a case shall be considered in which there is an error in the ejection volume of the i-th nozzle 902i, for example. As shown in FIG. 31, it is supposed that a dot 908i having a larger than ideal size is ejected from the i-th nozzle 902i. If the dot size becomes large in this way, then the density value which is obtained by measuring the density with the optical sensor 920 also increases, and the density of the regions where the dots 908i are ejected and hit becomes higher in comparison with the ideal value.
In this case, if the density measurement as described in Japanese Patent Application Publication No. 5-69545 is carried out, the density characteristics of the area corresponding to each nozzle (the micro density characteristics D) are shown on the graph on the right-hand side of FIG. 31. In other words, the measurement shows that the density of the area Di covered by the i-th nozzle 902i increases. In this case, it is possible to (physically) compensate the actual ejection volume by controlling the drive force on the basis of the measured density characteristics according to a method described in Japanese Patent Application Publication No. 5-69545, thereby compensating the non-uniformity of density. As a result, a flat graph of density measurement values as shown on the right-hand side of FIG. 32 can be obtained.
Furthermore, in this case, for example, a visual compensation is carried out by omitting the droplet ejection from the i-th nozzle 902i once in every five times in such a manner that the droplet ejection rate of the i-th nozzle 902i is controlled to 4/5 as shown in FIG. 33, and thereby it is also possible to suppress the density of the area Di covered by the i-th nozzle 902i and to obtain a flat density measurement graph as shown on the right-hand side of FIG. 33. Furthermore, in the method described in Japanese Patent Application Publication No. 2004-58282, it is possible to perform the similar compensation. In this way, according to the related art, it is possible to compensate non-uniformity of density arising from errors in the ejection volume of nozzles.
However, as described below, in the related art, there is a possibility that it is difficult to compensate non-uniformity of density caused by ink-landing position errors of the nozzles.
Here, a case where the dot diameter is smaller than the nozzle pitch as shown in FIG. 34 is described. For example, in cases where droplets are ejected to form small dots in a low-density region by a multiple-value inkjet printer capable of ejecting droplets to form dots of a plurality of dot sizes, droplets are ejected to form dots having a smaller dot diameter than the nozzle pitch in this way.
Here, a case where there is an ink-landing position error of the i-th nozzle 902i is described. More specifically, as shown in FIG. 34, the dots 908i ejected by the i-th nozzle 902i are displaced from the ideal droplet ejection positions indicated by the dotted line.
In this case, although the dots 908i formed by droplets ejected from the i-th nozzle 902i are displaced from the ideal landing positions as shown in FIG. 34, they are still located in the area Di covered by the i-th nozzle 902i and there is no change in the overall amount of coloring material inside this area Di. Hence, if the density is measured by the optical sensor 920 described above, then an even result of the density measurement is obtained with respect to the density characteristics of each nozzle, as in the graph shown on the right-hand side in FIG. 34. Thus the measurement show that there is no non-uniformity in density, and it is therefore difficult to carry out the compensation of the non-uniformity in density.
However, if the sample shown in FIG. 34 is viewed by a human observer with the naked eye, then the displacements are visible as linear non-uniformity. On the other hand, in the above-described related methods for compensating non-uniformity of density, a problem of this kind is not taken into account, and therefore it is difficult to carry out the sufficient compensation and such methods are not particularly effective with respect to non-uniformity of density arising from landing position errors.
The resolution of the optical sensor is equal to the nozzle resolution (the reciprocal of the nozzle pitch) in the examples described above; however, if the resolution of the optical sensor is set to a higher resolution, then it is possible to detect non-uniformity of density even in cases such as that shown in FIG. 34, in principle. For example, if the resolution of the optical sensor is doubled as shown in FIG. 35 and the measurement is carried out under the situation where the area Di covered by each nozzle 902i is divided into two parts, then a graph indicating density characteristics is obtained as shown on the right-hand side in FIG. 35, and hence the non-uniformity of density can be detected.
However, the density characteristics obtained in this way fluctuate in a range smaller than the nozzle pitch which is the smallest possible control unit. Hence, according to the non-uniformity of density compensation methods of the related art such as those described in Japanese Patent Application Publication Nos. 5-69545 and 2004-58282, it is difficult to compensate non-uniformity of density arising from ink-landing position errors as shown in FIG. 34.
It is not limited to the cases where the dot diameter is smaller than the nozzle pitch in this way. In general, there are cases where the dot diameter is larger than the nozzle pitch; however, the essence of the problems in these cases is the same as that described above. Hence, in such cases, it is also difficult to compensate non-uniformity of density which arises from ink-landing position errors.
In these cases, if the density characteristics are measured according to the method as described above, then a density variation depending on the surface area of the portion of the dot 908i which projects into the print area of the adjacent nozzle as shown in FIG. 36, for example, is measured as the density characteristics of each print area Di, and the corresponding nozzle is considered as a subject for the compensation. Here, the surface area of the portion of the dot which does not project into the adjacent print area is taken not to be a subject for the compensation, but in fact, it is distributed in an uneven fashion within the print area Di, for example, it is weighted toward the bottom within the print area Di, as shown in FIG. 37. Therefore this can become a cause of non-uniformity in density. However, according to the related art described above, this factor is not taken into account, and therefore the compensation may not be complete.